If you take a look at other translations, prophecy in NT is actually twofold. In 1 Cor. Paul is more likely talking about ministry which outsiders may come by and hear. OT on the other hand is more about fore-telling. It is so because Israel has the responsibility to identify a true prophet to have his words recorded down, as "God puts words in the mouth of a prophet". In NT it's more about how the gospel shall be preached in the absence of Israel as Israel literally no longer functions due to the fact that the corrupt Pharisees sit in Moses' seat while distancing God.
In NT, the Holy Spirit is poured out to many, so prophets are no longer the only ones dreaming dreams or prophesying.
Acts 2:17
In the last days, God says, I will pour out my Spirit on all people. Your sons and daughters will prophesy, your young men will see visions, your old men will dream dreams.
That said, prophets are about how God puts words into their mouth. More likely it's about what God does, it's much less about what the prophets do. God can communicate unambiguously to the prophets for them to comfortably use terms such as "says the Lord".
It is more likely in the manner that "If God asks me to prophesy then I prophesy, or else I won't", and "unless the message is made unambiguously to me, or else I won't prophesy".
If a prophet prophesies by his own will, I know that God doesn't like it, as it is more or less the behavior of Balaam working for his self-gain.